The present invention relates to a displacement control valve incorporated in variable displacement compressors that are used in vehicle air conditioners and to a method of manufacture. More particularly, the present invention relates to a displacement control valve that controls the difference between the pressure in a crank chamber and the pressure in cylinder bores, and includes a mechanism for changing a target suction pressure of the compressor.
A typical variable displacement compressor has a supply passage for connecting a discharge chamber with a crank chamber and a displacement control valve located in the supply passage. The displacement control valve controls the opening amount of the supply passage for adjusting the amount of highly pressurized refrigerant gas that is supplied to the crank chamber from the discharge chamber. The pressure in the crank chamber is changed, accordingly. This alters the difference between the pressure in the crank chamber and the pressure in cylinder bores. Changes in the pressure difference adjust the inclination of a swash plate of the compressor and ultimately change the displacement of the compressor.
Japanese Unexamined Patent Publication No 3-23385discloses such a displacement control valve 101 as illustrated in FIG. 7. The control valve 101 includes a housing 102 and a solenoid 111, which is secured to the bottom of the housing 102. The housing 102, together with an inner wall of the compressor, defines a high pressure chamber 109. The housing 102 also includes a low pressure chamber 107 defined in its lower portion and an intermediate pressure chamber 110 located between the chambers 109 and 107. The low pressure chamber 107 accommodates a bellows 108. A valve seat 103 is located between the high pressure chamber 109 and the intermediate chamber 110. The valve seat 103 has a valve hole 104. The upper end of the bellows 108 is coupled to a rod 106, which extends through the valve hole 104. The distal end of the rod 106 is coupled to a valve body 105, which faces the valve seat 103 to open and close the valve hole 104. In other words, the rod 106 connects the valve body 105 with the bellows 108. The low pressure chamber 107 communicates with suction pressure Ps of the compressor. The suction pressure Ps therefore expands or collapses the bellows 108. The high pressure chamber 109 communicates with a discharge chamber of the compressor by the upstream portion of the supply passage. Therefore, the discharge pressure Pd is introduced to the high pressure chamber 109. The intermediate pressure chamber 110 communicates with the high pressure chamber 109 by the valve hole 104 and is connected to the crank chamber by the down stream portion of the supply passage.
A solenoid 111 is secured to the bottom of the housing 102. A fixed steel core 113 is provided at the upper portion of the solenoid 111. A steel plunger 112 is arranged in the solenoid 111 and moves along the axis of the plunger 112. A rod 112a is coupled to the plunger 112 and extends through the core 113. A coil 114 is wound about the plunger 112 and the fixed core 113. The top end of the rod 112a is attached to the inner wall of the bellows 108. A spring 115 extends between the bottom end of the plunger 112 and the bottom of the solenoid 111. The spring 115 urges the plunger 112 upward. That is, the spring 115 urges the valve body 105 in a direction separating the valve body 105 from the valve seat 103 to open the valve hole 104.
An external control unit (not shown) sends electric current to the coil 114. The magnetic attractive force produced between the plunger 112 and the fixed core 113 is varied by the magnitude of the current from the control unit. The magnitude of the force that pushes the plunger 112 upward, or the force for separating the valve body 105 from the valve seat 103, corresponds to the magnitude of the attraction force. When the solenoid 111 is excited, increasing the suction pressure Ps contracts the bellows 108 and lowers the plunger 112. This causes the valve body 105 and ultimately closes the valve hole 104. Contrarily, lowering the suction pressure Ps expands the bellows 108 and lifts the valve body 105. This opens the valve hole 104. In this manner, the opening area between the valve body 105 and the valve hole 104 is adjusted in accordance with the suction pressure Ps. The level of the suction pressure Ps required for lowering the valve body 105, that is, for moving the valve body 105 toward the valve seat 103, is varied in accordance with the attraction force produced between the armature 112 and the retainer 113.
The above described prior art control valve 101 has the following disadvantages.
If a vehicle having the above compressor, which is connected to an external refrigerant circuit, is caught in a traffic jam in summer, the heat exchange capacity of the condenser in the circuit is significantly lowered. In this case, the valve body 105 closes the valve hole 104 and the compressor operates at the maximum displacement. This results in an extremely high discharge pressure Pd and causes the pressure Pc in the crank chamber to approach the lower suction pressure Ps. In this state, the upper surface of the valve body 105 receives the high discharge pressure Pd and the lower surface of the valve body 105 receives the pressure in the intermediate pressure chamber 110, or the pressure Pc in the crank chamber. A force based on the difference between the pressures Pd and Pc strongly presses the valve body 105 against the valve seat 103. The valve body 105 is therefore not easily moved in a direction to open the valve hole 104 and the responsiveness of the valve body 105 to the suction pressure Ps is degraded In other words, the valve body 105 does not respond to subtle changes in the auction pressure Pa.
If the cooling load falls when the compressor is operating at the maximum displacement, the displacement must be decreased. In order to decrease the displacement, the opening area between the valve body 105 and the valve hole 104 must be enlarged. The valve body 105 thus must be moved by a force that is greater than the force resulting from the difference between the discharge pressure Pd and the crank chamber pressure Pc. That is, the attractive force between the plunger 112 and the fixed core 113 must be increased for enlarging the opening area between the valve body 105 and the valve hole 104. In order to increase the attractive force, the solenoid 111 must be larger. A large solenoid 111 consumes a relatively large amount of electric power and thus increases the load on the alternator.